The many advantages of flourescent lighting (e.g., low power consumption, long life) have, in recent years, spurred development of battery operated lamps using inverters (say, for camping) and the compact flourescent light fixtures intended to be screw-in replacements for standard incandescent light bulbs. Even line operated applications where, in the past, the venerable magnetic ballast would have been employed (e.g., permanently installed overhead lighting fixtures in structures), the magnetic ballast has begun to see its replacement by electronic ballasts (which, of course, are also found in the compact flourescent replacements for incandescent lamps). Accordingly, there has been much commercial interest in electronic ballasts. For a general introduction to what a ballast needs to do and the various approaches to such ballasts, see the eminently readable and informative Motorola application note AN1049D (1990, 1994). Additional background may be gained from the incorporated Patents. The subject matter of the present invention concerns what that application note terms a "voltage fed resonant circuit" type of electronic ballast.
Despite all the development that has gone on, there is still room for improvement. First, the circuit should be efficient. Efficiency has many implications. The reduced heat allows longer component life and increased freedom of circuit deployment to product situations that might not otherwise be possible. The flourescent bulbs themselves seem to produce more light for a given power input thereto when the applied power is sinusoidal and of fairly high frequency, say, 50 KHz. Unfortunately, the sine wave needed for, say, an F40T 12 is substantial; perhaps 750 VRMS to start it and approximately hall that to keep it going once started. It is not trivial to keep distortion in the sine wave small under such conditions, especially since the tube is not a simple resistive load. Finally, if no special mechanism is included to preheat the electrodes to assist in conditioning the mercury vapor in the tube, the tube can be hard to start, or can resist the transition from a start-up phase to an operating phase. It would be desirable if the conventional voltage fed electronic ballast could be improved to be more efficient, have a less distorted sine output, while starting reliably without resort to any separate "starters" for preheating the electrodes.